Hereditary deficiency of galactose-1-phosphate uridylyltransferase (GALT, E.C. 2.7.7.12) activity in humans can lead to a potentially lethal disease called Classic Galactosemia (OMIM 230400). Despite the life-saving consequences of newborn screening, early diagnosis, and a galactose-restricted diet, many patients with Classic Galactosemia suffer later in life from complications including growth, neuropsychological, and speech delays as well as primary ovarian insufficiency (POI). There are currently no satisfactory treatments available to prevent/alleviate any of these complications. Several lines of evidence, which include the absence of acute toxicity syndrome and chronic complications in patients with inherited deficiency of galactokinase (GALK1), as well as the reversal of glycosylation/neurological defects in GALT-less fruitflies by deleting the dGALK gene, indicated that galactose-1 phosphate (gal-1P), product of GALK1 and an intermediate accumulated in GALT deficiency, is toxic in susceptible tissues and plays a pathogenic role in the organ- specific complications of Classic Galactosemia. Therefore, we hypothesize that pharmacological inhibition of human GALK1, which will prevent the accumulation of toxic gal-1P, can confer the less severe phenotype of GALK1 deficiency and improve the outcome of Classic Galactosemia. To test this hypothesis, we collaborated with scientists at NIH/NCATS/NCGC and launched a quantitative high-throughput screening (qHTS) campaign to identify small molecule inhibitors of human GALK1. After extensive characterization and optimization efforts of the positives, we have identified some unique and promising lead GALK1 inhibitors with dissociation constants (a measure of the strength of the inhibitors) at the nanomolar range, as well as favorable cell-based and in vivo activities. The translation of these promising small molecule GALK1 inhibitors into useful therapeutics, however, requires further chemical modifications and in vivo studies in a mammalian animal model. To prepare for the testing of the preclinical in vivo studies of selected GALK1 inhibitors, we have constructed a new GalT-gene trapped (GALT-deficient) mouse model. Expanded characterization of the new mouse model revealed disease-relevant phenotypes, which include galactose sensitivity in the newborn mutant pups, subfertility of the adult female mutants, motor impairment and growth restriction in mutant mice of both sexes. Specifically, in Aim 1 of this application, we propose to further optimize the selected GALK1 lead inhibitors to improve their biological activities and drug-like properties; and in Aim 2, we will evaluate the in vivo efficacy of the selected GALK1 inhibitors in ameliorating selected disease-relevant phenotypes in the homozygous GalT-gene trapped mice.